1. Field of the Invention
The present invention relates to an optical system in which an optical material with extraordinary partial dispersion is used, or an optical system that is suitable for an image taking optical system of a silver halide film camera, a digital still camera or a video camera, or a projection optical system of a liquid crystal projector.
2. Description of the Related Art
Conventionally, a retrofocus type lens has been known as a type of lens having a short focal length and a long back focus. In the retrofocus type lens, a long back focus can be realized by arranging a lens unit having negative refractive power as a whole in the front side (i.e. the magnification side: the object side in the case of a image taking optical system of camera and or the like, or the screen side in the case of a projection optical system of a projector or the like) of the optical system and arranging a lens unit having a positive refractive power as a whole in the rear side (i.e. the reduction side: the image side in the case of a image taking optical system of a camera or the like, or the original side in the case of a projection optical system of a projector or the like) of the optical system. In order to obtain a long back focus, it is necessary to enhance both the positive refractive power and the negative refractive power, which results in an optical system having an asymmetrical refractive power arrangement. Problems the retrofocus type lens suffers in correcting aberrations are that barrel type distortion is liable to occur, that significant chromatic aberration of magnification (lateral chromatic aberration) is liable to occur, and that secondary spectrum of chromatic aberration of magnification tends to become large.
As a conventional method for improving the chromatic aberration of magnification, a method of using a low dispersion lens made of extraordinary partial dispersion material such as fluorite and a method of using a diffraction optical surface have been proposed.
On the other hand, diffraction optical elements have an Abbe number equivalent value of as small as 3.45 (in the absolute value), and accordingly it is possible to change chromatic aberration greatly while scarcely affecting spherical aberration, coma and astigmatism etc. only by slightly changing the optical power (which is the reciprocal of the focal length) achieved by diffraction. In addition, since diffracted light is used, the optical power changes linearly depending on the wavelength of incident light, and the wavelength characteristics of the chromatic aberration coefficient is completely linear. Therefore, in order to shorten the total length of the system, aberration corrections should be mainly directed to spherical aberration, coma and astigmatism. As to chromatic aberration, since correction thereof is effected using a diffraction optical element, what is required in designing the system is only to optimize the material and optical power of the constituent lenses so as to realize linearity in the wavelength characteristics of the chromatic aberration without taking into consideration the absolute amount of the chromatic aberration that has been deteriorated by the length reduction. Thus, an optical system having excellent performance can be obtained as a consequence.
It has been proposed to use a resin material mixed with inorganic oxide fine particles such as ITO fine particles in a diffraction grating to improve the diffraction efficiency (see Japanese Patent Application Laid-Open No. 2001-074901 (a counterpart: EP A2 1065531).
Furthermore, a liquid material having relatively high dispersion and showing relatively extraordinary partial dispersion characteristics has been known as a material having a chromatic aberration correction effect similar to diffraction gratings, and an achromatic optical system using the same has been proposed (see U.S. Pat. No. 4,913,535).
Since the refractive index of a low dispersion lens with extraordinary partial dispersion such as a fluorite lens is low, its position in an optical system is limited when used, or it is sometimes necessary to increase the number of the lenses in the system. In addition, such lenses are very expensive and they cannot be used frequently in view of the cost.
Although diffraction optical elements have sufficient chromatic aberration correction effects, they suffer from the problem that diffracted light of useless diffraction orders different from the designed diffraction order becomes colored flare light that deteriorates imaging performance. Although in some cases, a so-called laminated diffraction optical element composed of a plurality of blazed diffraction gratings stacked along the optical axis direction is used so as to concentrate energy on the designed diffraction order and to reduce useless diffracted light greatly, there still remains the problem that diffracted flare is generated when an object having high luminance is photographed.
In manufacturing a diffraction optical element, a method of molding an ultraviolet curing resin or the like using a metal mold has been known. However, diffraction efficiency of a diffraction optical element is extremely sensitive to the manufacturing accuracy, and a very high degree of precision in the metal mold and in the molding process is required. This leads to high manufacturing costs.
The material disclosed in U.S. Pat. No. 4,913,535 is liquid, and a structure for sealing it is necessary and manufacturing thereof is not easy. In addition, it suffers from the problem of changes in characteristics, such as refractive index and dispersion characteristics, depending on the temperature, namely, it does not have sufficient environmental tolerance. In addition, that material suffers from the defect that it cannot achieve a sufficient chromatic aberration correction effect due to its relatively large Abbe constant, relatively small extraordinary partial dispersion and absence of interface with the air.